1. Field of the Invention
This invention relates to control apparatus in a stored program computer and more particularly to instruction processing control apparatus.
The present invention finds particular utility in computer systems which have been modified to expand the function thereof by addition of control registers. These additional control registers can be loaded or sensed by the present invention without constructing special storage to register instructions. In a sense the present invention extends register addressing capability in an existing computer system without expanding the size of the instructions. Hence, the added control registers can be addressed for transfer of data thereto or therefrom. The present invention uses substantially the same decode circuitry as used for an existing storage control instruction. Hence, the present invention eliminates extensive additional decode circuitry which would be otherwise required if special storage to register instructions were used.
The registers added to expand the function of the computer system could be, for example, to provide address translation. If address translation is provided, mode control registers might be added to control the translate mode. Normally a special instruction would be required to load or sense the address translation registers and another special instruction would be required to load or sense the mode control registers. The register control instruction of the present invention can load or sense the address translation registers and the mode control registers.
2. Prior Art
Heretofore, it has not been the practice to use the same OP code for two different instructions and inhibit the operation specified by the OP code and perform a different operation using substantially the same control circuitry as used for the inhibited operation. Use of predecoders and function modifiers is well known in the prior art as represented by U.S. Pat. Nos. 3,657,705 and 3,889,242. The prior art, as represented by a publication in the IBM Technical Dislosure Bulletin, Vol. 8, No. 12, May, 1966, Page 1751, also teaches expanding an instruction set by providing a special instruction within the basic instruction set which switches the system into an alternative mode where an instruction in one mode has one meaning and another meaning in the alternative mode. Hence, not only are special instructions required, but new control circuitry responsive to the instruction meaning in the alternative mode is also required.